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From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Nadav Amit <namit@vmware.com>
Cc:     Mauro Carvalho Chehab <mchehab@kernel.org>,
        "linux-media@vger.kernel.org" <linux-media@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] usb: fix uvc_alloc_entity() allocation alignment
Date:   Tue, 07 Aug 2018 11:59:01 +0300
Message-ID: <3561796.imRZE4xQI7@avalon>
Organization: Ideas on Board Oy
In-Reply-To: <0B044CD5-B4F5-4614-B97A-E02E5C1E8A17@vmware.com>
References: <20180604134713.101064-1-namit@vmware.com> <15813968.YrTFj7ZbY9@avalon> <0B044CD5-B4F5-4614-B97A-E02E5C1E8A17@vmware.com>
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Hi Nadav,

On Tuesday, 7 August 2018 03:58:05 EEST Nadav Amit wrote:
> at 4:58 PM, Laurent Pinchart <laurent.pinchart@ideasonboard.com> wrote:
> > On Monday, 4 June 2018 16:47:13 EEST Nadav Amit wrote:
> >=20
> >> The use of ALIGN() in uvc_alloc_entity() is incorrect, since the size =
of
> >> (entity->pads) is not a power of two. As a stop-gap, until a better
> >> solution is adapted, use roundup() instead.
> >>=20
> >> Found by a static assertion. Compile-tested only.
> >>=20
> >> Fixes: 4ffc2d89f38a ("uvcvideo: Register subdevices for each entity")
> >>=20
> >> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> >> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> >> Cc: linux-media@vger.kernel.org
> >> Cc: linux-kernel@vger.kernel.org
> >>=20
> >> Signed-off-by: Nadav Amit <namit@vmware.com>
> >> ---
> >> drivers/media/usb/uvc/uvc_driver.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>=20
> >> diff --git a/drivers/media/usb/uvc/uvc_driver.c
> >> b/drivers/media/usb/uvc/uvc_driver.c index 2469b49b2b30..6b989d41c034
> >> 100644
> >> --- a/drivers/media/usb/uvc/uvc_driver.c
> >> +++ b/drivers/media/usb/uvc/uvc_driver.c
> >> @@ -909,7 +909,7 @@ static struct uvc_entity *uvc_alloc_entity(u16 typ=
e,
> >> u8 id,
> >>  	unsigned int size;
> >> 	unsigned int i;
> >>=20
> >> -	extra_size =3D ALIGN(extra_size, sizeof(*entity->pads));
> >> +	extra_size =3D roundup(extra_size, sizeof(*entity->pads));
> >> 	num_inputs =3D (type & UVC_TERM_OUTPUT) ? num_pads : num_pads - 1;
> >> 	size =3D sizeof(*entity) + extra_size + sizeof(*entity->pads) * num_p=
ads
> >> 	     + num_inputs;
> >=20
> > The purpose of this alignment is to make sure that entity->pads will be=
=20
> > properly aligned. In theory the size of uvc_entity should be taken into=
=20
> > account too, but the structure contains pointers, so its size should
> > already be properly aligned. This patch thus looks good to me. What
> > made you say it's a stop-gap measure ?
>=20
>=20
> Thanks. It=E2=80=99s been a while. Anyhow, I don=E2=80=99t know how =E2=
=80=9Chot=E2=80=9D this code is, but
> roundup uses a div operations, so if it is =E2=80=9Chot=E2=80=9D you may =
want a different
> way to align with lower overhead.
>=20
> I presume it is not=E2=80=A6

You're right, it isn't. I'll include this patch in my next pull request for=
=20
v4.20.

=2D-=20
Regards,

Laurent Pinchart